1. Statement of the Technical Field
The inventive arrangements relate to antennas, and more particularly to antennas having very wide impedance bandwidth and pattern bandwidth.
2. Description of the Related Art
For design purposes, an antenna can be thought of as having two ports. During transmission, the first port (which is commonly referred to as the antenna feed port) allows energy to flow from a transmission line into the antenna. The transmission line will have some characteristic impedance, such as 50Ω. The second port can be envisioned as allowing energy to flow from the antenna into free space. Free space has a characteristic impedance of 120/πΩ.
An antenna is sometimes loosely described in terms of its “bandwidth.” Bandwidth usually refers to the range of operating frequencies over which the antenna is designed to provide some level of satisfactory performance. However, there are actually at least three different performance characteristics that define an antenna's useful operating frequency range. Specifically, antenna performance is appropriately considered with regard to input impedance, gain and radiation pattern, each of which can serve to limit the useful bandwidth or operating frequency range of the antenna. For example, it is usually desirable for antenna input impedance to be maintained within a desired range so that the input VSWR for the antenna is less than about 3:1. The useful bandwidth of an antenna can be limited when the input impedance is outside of such desired range. Likewise, an antenna may not have sufficient gain outside a certain operating frequency range and this factor can limit the useable bandwidth of the antenna. Finally, an antenna can exhibit an undesirable radiation pattern at certain frequencies, and this too can limit the useful bandwidth of the antenna.
Inductors have long been used within antennas to make the antenna appear electrically longer, or stated another way, to make a physically short antenna (<0.5λ) appear electrically resonant. The ideal inductor placement and size is a function of practicality versus functionality. The most efficient implementation of such antennas is an infinitely large inductor at the element end opposed from the feed. This results in a uniform current distribution over the entire length of the antenna element. Conversely, the most practical implementation is generally a small inductor placed near the feed system. This results in a uniform current distribution from the feed to the inductor and then triangular distribution to the element end. Between these limiting cases, the inductor size grows as the inductor is placed further away from the feed point. A trade-off analysis can be performed to determine the most advantageous inductor size and placement to make a short antenna appear electrically resonant in a particular application.